Monitoring circuit for control means and selective breakaway means in modular supply systems

ABSTRACT

Modular supply systems usually include a plurality of n elementary power units, connected in parallel to each other. Control unit (RV) and selective breakaway unit (DS) are associated with each elementary power unit. The monitoring circuit (CP) includes a first device (PM) which calculates the entity of the deviation (E) between a signal (Iu i ) that is proportional to the current supplied by the relevant power unit (UA i ) and a signal (I med ) that is proportional to the average value of the currents supplied by the n power units. Also, a second device (SM) algebraically sums the deviation (E) to a signal (Vu i ) that is proportional to the voltage available on the output of the relevant power unit. The signal (Vc) that is available on the output of the second device is delivered to the control unit (RV) and the selective breakaway unit (DS).

BACKGROUND OF THE INVENTION

The present invention relates to a monitoring circuit for control meansand selective breakaway means in modular power supply systems,particularly for telecommunications equipment. More particularly, thepresent application relates to a circuit designed to enable the rightdistribution of the load among the elements which compose the system aswell as the location and the selective switching off of the units thatare out of order. The supply systems of the type described above areusually comprised of a plurality of basic unit (such as rectifiers, orinverters, etc.) that are connected to each other in parallel. This typeof connection is used, for example, when the power to be supplied by thesystem is not delivered by a single power unit and the power istherefore supplied by several power units that are connected in parallelto each other. In addition to the power units that are necessary tosupply the operating power, usually at least one other power unit isadded, in order to act as a hot reserve to be used in case of a failureof one or more of the operating power units.

In these types of systems, optimum conditions are obtained when thetotal current absorbed by the load is equally supplied by the nelementary units, that is when each power unit supplies a current Ihaving a value of It/n.

Furthermore, it must be noticed that when the power units are comprisedof inverters, besides the optimization problems as described above,further new technical problems occur in connection with the phaserelationship of the oscillations that are supplied by each inverter. Infact, an inverter is constituted by a unit that is designed to receive acontinuous voltage on its input and the inverter is adapted to deliver asinusoidal voltage to the output so that the frequency of the inverternormally coincides with the frequency of the electrical powerdistribution line.

The inverters of the previous generation are characterized by the factthat they present an output impedance that does a not have anexcessively low value. Consequently, the problems relating to an equalpower distribution as described above can be solved through slow controladjustment systems.

On the other hand, modern generation inverters are configured toapproximate an ideal voltage generator and are consequentlycharacterized in that they have a particularly low output impedance. Thelow impedance values result from the use of means that are designed toact in such a manner that the oscillations generated by the invertershave the same level and phase at any time. In fact, if the power systemis constituted by n elementary units, having one unit which supplies anoscillation that is shifted in phase with respect to the oscillationsgenerated by the further n-1 units, the power delivered by the n-1 unitsis delivered to the unit that is out of phase. Consequently possibledamage to the unit may be caused the power supplied to the load may bereduced. CL SUMMARY OF THE INVENTION

Therefore, the basic technical problem of the present invention is howto realize a supply system with a monitoring circuit that isparticularly fast for allowing level and phase variations in theoscillations, generated by each inverter, in a time as short aspossible. A short time will minimize the duration of the negativeeffects caused by a possible phase shift as described above and willconsequently avoid the danger of any damage. The Purpose of theembodiments in the present application is to solve the aforementionedtechnical problem through the use of circuitry that is particularlysimple and economical.

For this purpose and according to the present embodiments of the supplysystem is of the type comprising n elementary power units, that areconnected in parallel to each other with each having control andselective breakaway means.

The monitoring circuit includes the presence of the followingcharacteristic elements:

First means, designed to receive a first signal on their input, that isproportional to the current supplied by the relevant power unit. Also asecond signal that is proportional to the average current value issupplied by the n power units and is adapted to calculate the entity ofan error signal corresponding to the difference between the first signaland the second signal;

Second means, designed to algebraically add the error signal at theoutput of the first means to a third signal. The value of the thirdsignal is proportional to the voltage that is supplied by the relevantpower unit. Furthermore, the second means is designed to send the thirdsignal, obtained as described above, to the control and selectivebreakaway means.

The control means modifies the level of the signal relevant power unituntil the error signal is cancelled. On the other hand, the selectivebreakaway means switched off the relevant power unit from the parallelconnection when the signal that is supplied by the second means deviatesfrom a nominal value by a predetermined entity.

BRIEF DESCRIPTION OF THE DRAWING

Further characteristics of the application will be more evident in thefollowing description only one embodiment of the invention is given asan example and is complemented by the attached drawings in which:

FIG. 1 shows a block diagram of a supply system that is referred to inone embodiment of the present application.

FIG. 2 shows in detail one of the monitoring circuits CP from FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, UA₁, UA₂, . . . , UA_(n) refer to a plurality of power unitsthat are connected in parallel to each other by means of the switchesT₁, T₂, . . . , T_(n). A monitoring circuit CP, voltage control means RVand selective breakaway means DS are associated to each power unit UA.

Each monitoring circuit CP is connected by means of a contact K of therelevant switch T to a common bus B. There is a signal I_(med) on thecommon bus B that is proportional to the average value of the currentsupplied by each power unit UA.

Furthermore, the circuits CP receive a signal Iu_(i) on their inputsthat is proportional to the current value supplied by the relevant powerunit UA_(i) that is and a signal Vu_(i) proportional to the voltagesupplied by the relevant power unit UA_(i). Each signal IU_(i) isrespectively detected by means of a voltage transducer TV_(i).

The task of each monitoring circuit is to generate a signal VC inresponse to the presence of a voltage error on the relevant power unitUA of the same value.

In fact, the signal Vc of reaches the input of the respective voltagecontrol means RV which modifies the level of the signal generated by therelevant power unit UA. The signal Vc is modified until a nominal valueis reached and consequently the signal generated by the relevant powerunit has a value that is within the predetermined tolerance limits.

Also, the signal Vc reaches the input of the respective selectivebreakaway means DS which activates on output when the difference betweenthe signal Vc and nominal value of the respective selective breakawaymeans evidences the presence of a failure in the relevant power unit UA.Considering that a generical power unit can be considered to be out oforder when the output voltage differs by more than 5% from a nominalvalue, the means DS has the task of detecting the presence of thiscondition and activating an output in order to determine the opening ofthe relevant switch T_(i). Thus, is switched off the unit UA that hasfailed.

FIG. 2 illustrates a generical monitoring circuit CP which comprises byfirst means PM adapted to calculate the entity of the error signal E onthe relevant power unit UA. Furthermore, second means SM algebraicallysums the error signal E to a signal that is proportional to the voltagesupplied by the relevant power unit UA.

The first means PM comprises by an operational amplifier AO₁, to theinputs of which the signal Iu_(i) (which is proportional to the currentsupplied by the relevant power unit) is applied by means of first and asecond resistors R₁ and R₂ of equal value. Between the output of theamplifier A0₁ and the inverting input, a first feedback resistor R₃ isconnected. While between the non-inverting input and ground, a secondfeedback resistor R₄ is connected having a value equal to that of theresistor R₃. When a generical power unit UA_(i) is inserted into theparallel connection, the switch T_(i) and the relevant contact K_(i) areclosed. As a consequence, the signal I_(med), which is proportional tothe average value of the current supplied by the power units, reachesthe non-inverting input of the unit AO₁.

If the resistors described above are so dimensioned as to satisfy therelationship R₁ =R₂ and R₃ =R₄, an error signal E, expressing thevoltage error on the relevant power unit corresponds to the output ofthe unit AO₁. In particular, the error signal E represents a signal thatis proportional to the entity of a shifting between the voltage suppliedby the power unit actually under examination and the average value ofthe voltage supplied by the remaining (n-1) units.

On the other hand, the means SM comprises by a second operationalamplifier AO₂ which has the non-inverting input connected to a referencepotential and the inverting input connected to the output of the unitAO₁ by means of a resistor R₅. The inverting input also receives thesignal Vu_(i) by means of a resistor R₆ and a feedback signal by meansof a resistor R₇. Under normal operating conditions the output voltagesof the various power units are perfectly pulled in phase. The outputvoltage VU that is applied to the load Z is equal to the average valueamong the voltages at the outputs of the varius power units. Each powerunit compares its own current output with the average current andadjusts its own output voltage until a current equal to the averagevalue is supplied.

In the event that one of the power units delivers a higher current thanthe average current, the error signal E_(i) will appear to be in phaseto the output voltage Vu_(i). Consequently, the regulation means RV_(i)receives a signal VC_(i) on the input with a higher value than thenominal one. As a consequence, the means RV_(i) arranges a reduction inthe current supplied by the relevant power unit UA_(i) until the currentvalue is approximately equal to the average current. On the contrary, ifone of the power units supplies a lower current than the average currentthe error signal E_(i) will appear to be in phase opposition to thesignal Vu_(i) and consequently the regulation means RV_(i) receives asignal Vc_(i) on the input having a lower value than the nominal signal.As a consequence, the means RV_(i) determines an increase in the currentsupplied by the relevant power unit UA_(i) until the current value isapproximated equal to the average current.

Besides allowing the adjustment of the current that is supplied by therelevant power unit, the signal Vc also enables indications about therelevant power unit operation.

In fact, the weight of the signal E_(i) has been determined in such away that the power units which operate at nominal voltage, receive asignal Vc having a nominal value on the inputs. In particular, theweight of the error signal E_(i) has been determined so that the errorthat is picked up on the signal Vu_(i), due to the failure of a powerunit, is the same vaue but opposte to the contribution brought by thesignal E_(i). Consequently the signal Vc, which refers to the unitsoperating at nominal voltage, presents a nominal value.

In contrast the faulty unit the error present on the signal Vu_(i) inconsequence to the failure of the power unit, is added to the errorE_(i). Consequently, the selective breakaway means DS_(i) of the faultyunit UA_(i) receives a signal Vc on the input that differs from thenominal value for the same error percentage that is present on theoutput voltage.

Let us suppose that the power system is composed of two elementary powerunits, the first of which is operating at nominal voltage, while thesecond presents an overvoltage error equal, for example, to 5%. If theoutput voltage is supplied by inverters having a low output impedance,then the voltge VU that is applied to the load will be equal to VU_(n)(1+2.5%), where VU_(n) represents the nominal output voltage.

On the basis of the material specified above, the voltage regulationmeans RV of the inverter that is not faulty, receives a signal Vc on theinput having the same value as the nominal signal Vc=Vu_(n), whereVu_(n) represents a signal that is proportional to the nominal outputvoltage VU_(n). In fact:

    (Vu+2.5% VU.sub.n)+(-2.5% Vu.sub.n)=Vu.sub.n =Vc.

This is due to the fact that the signal E_(i) appears to be in anopposite phase to the signal Vu_(i) and the contribution brought byE_(i) appears to be the same value but opposite to the variation presenton the signal VU_(i).

The voltage control circuit RV of the inverter affected by the failurereceives a signal Vc on the input that differs from the nominal valueVc_(n) for the same error percentage that is present on the relevantpower unit. In fact:

    (Vu.sub.n +2.5% Vu.sub.n)+(+2.5% Vu.sub.n)=Vu.sub.n +5%Vu.sub.n =Vc.

This is due to the fact that the signal E_(i) is in phase with thesignal Vu_(i).

The selective breakaway means DS will not be described in detail hereinas they are of a well known type that comprises threshold circuitsdesigned to operate their outputs whenever signals are recieved on theirinput which differs from their own nominal value by a predeterminedentity (for example 5%).

The activation of the output of the means DS_(i) opens the switch T_(i)which switches off the power unit UA_(i) affected by the failure.

The voltage control means RV will be not described in detail as they arewell known to a person skilled in the art.

The monitoring circuits CP as claimed in the embodiment satisfy therequisites cited in the invention because they deliver the signal Vc,which allows the selective breakaway operations, to be carried out evenwhen the output voltage deviates from the nominal value by a limitedentity (for example ±5%). This has the advantage that the oversizing incurrent of the supply system, due the to current distribution, can becontained within low values (5% of the nominal current). In the controlsystems, already known, an oversizing of current to the order of 10% isrequired consequently, when the power system has to supply a higherpower (for example 100 KW), a 5% reduction of the oversizing powerconstitutes a value that is ignored.

Furthermore, the monitoring circuit according to the embodiment, ischaracterized by the fact that an adjustment of the output voltage isallowed in a very short time. Thus, the monitoring circuit is suitablefor use in combination with all types of power units, such asrectifiers, positive/negative voltage boosters or inverters.

Although the present invention has been described in connection with thepreferred embodiment thereof, many variations and modifications may bemade without departing from the scope or spirit of the invention. It ispreferred, therefore, the present invention is not limited by thedisclosure herein, but by the claims.

What is claimed is:
 1. A modular supply system for controlling powersupplied to a load, comprising:a plurality of power units connected inparallel, each of said plurality of power units including,power supplymeans for supplying a voltage signal and a current signal to the load,first monitoring means for developing an error signal corresponding tothe difference between said current signal and a source current signal,second monitoring means for developing a voltage error signalcorresponding to the summation of said voltage and said error signal,control means for regulating said voltage and current signals suppliedfrom said power supply means within a predetermined tolerance limit inresponse to said voltage error signal, and selective breakaway means fordisconnecting said power supply means in response to said voltage errorsignal being detected outside of said predetermined tolerance limit;each of said plurality of power units that has said power supply meansconnected providing an equal distribution of power to the load.
 2. Amodular supply system as described in claim 1, wherein said firstmonitoring means comprises:a first operational amplifier having saidcurrent signal connected by a first and a second resistor of equalvalues, and said source current signal connected to the non-invertinginput by a circuit breaker; a first feedback resistor connected betweenthe output of the first operational amplifier and the inverting input ofsaid first operational amplifer; and a second feedback resistor, havinga value equal to said first feedback resistor, connected between thenon-inverting input of the first operational amplifier and a referencepotential.
 3. A modular supply system as described in claim 1, whereinsaid second monitoring means comprises:a second operational amplifier,the non-inverting input of said second operational amplifier beingconnected to the reference potential and the inverting input of saidsecond operational amplifier being connected to the output of the firstoperational amplifier (AO₁), by a third resistor and said voltage signalbeing conncted by a fourth resistor; and a third feedback resistorconnected between the output of the second operational amplifier and theinverting input of said second operational amplifier.